The present disclosure relates to articles that incorporate holograms, more particularly volume transmission and reflection holograms. Methods of making and using the same are also disclosed.
Holograms are an increasingly popular mechanism for the authentication of genuine articles, whether it is for security purposes or for brand protection. The use of holograms for these purposes is driven primarily by the relative difficulty with which they can be duplicated. Holograms are created by interfering two coherent beams of light to create an interference pattern and storing that pattern in a holographic recording medium. Information or imagery can be stored in a hologram by imparting the data or image to one of the two coherent beams prior to their interference. The hologram can be read out by illuminating it with beams matching either of the two original beams used to create the hologram and any data or images stored in the hologram will be displayed. As a result of the complex methods required to record holograms, their use for authentication can be seen on articles such as credit cards, software, passports, clothing, and the like. In addition, the inherent properties of holograms (vivid coloration, 3-dimensional effects, angular selectivity, etc.) have long attracted the interest of artists and advertisers as a medium for generating eye-catching displays for commercial or private use.
Two categories of holograms include surface relief structure holograms and volume holograms. Many of the holograms used in display, security or authentication applications are of the surface relief type, in which the pattern and any data or image contained therein is stored in the structure or deformations imparted to the surface of the recording medium. While the initial holograms may be created by the interference of two coherent beams, duplicates can be created by copying the surface structure using techniques such as embossing. The duplication of holograms is convenient for the mass production of articles such as credit cards or security labels, but it also has the disadvantage that it makes the unauthorized duplication and/or modification of these holograms for use in counterfeit parts possible from the originals using the same mechanism.
Unlike surface holograms, volume holograms are formed in the bulk of a recording medium. Volume holograms have the ability to be multiplexed, storing information at different depths and different angles within the bulk recording material and thus have the ability to store greater amounts of information. In addition, because the pattern which makes up the hologram is embedded, copying cannot be done using the same techniques as for surface relief holograms. In addition, surface holograms are inherently polychromatic (rainbow-appearance), while volume holograms are capable of both monochromatic (at a desired wavelength) as well as polychromatic (either multicolored or rainbow-appearance), which enables greater control of the aesthetic features of volume holograms for display applications versus surface holograms.
While volume holograms can provide greater security against counterfeit duplication and greater aesthetic breadth than surface relief structure holograms, they generally require vibration-isolated, temperature-controlled recording equipment that must be maintained at physical tolerances of less than the writing light wavelength, typically on the order of hundreds of nanometers (e.g., 405 nm) in order to record well-defined, high diffraction efficiency holograms. Additionally, the laser sources, especially those used for traditional transmission holography in thick materials, must have long coherence lengths (e.g., centimeters to meters). All of this contributes to relatively high equipment costs for recording volume holograms. Accordingly, volume holograms have proven to be more time-consuming and expensive to mass produce because in many cases each holographic article must be individually exposed with interfering signal and reference light sources in order to produce the interference fringe patterns to create the holographic image. Mass production is even more problematic if it is desired to individualize or personalize individual holographic images, as the signal light source must be provided with different image information for each individualized holographic recording, which adds to the time, expense, and complexity of the holographic recording process. For example, individualized information such as photos, logos, serial numbers, images, and the like is often collected and/or maintained in a decentralized fashion at disparate locations, which would then require holographic recording equipment to be maintained and operated at a number of different locations, further adding to the required time, capital expense, and complexity.
Accordingly, there exists a need for new techniques for recording volume holograms that offer improved efficiency and/or lower cost. There also remains a need for new techniques for recording volume holograms with individualized images, information, or characteristics at improved efficiency and/or lower cost.